1. Field of Invention
The invention relates to catalyst degradation determining apparatus and method for determining whether a catalyst disposed in an exhaust passage of an internal combustion engine has degraded.
2. Description of Related Art
A known three-way catalyst (sometimes referred to simply as “catalyst” in this specification) for controlling exhaust gas from an internal combustion engine is disposed in an exhaust passage of the internal combustion engine. The three-way catalyst has an O2 storage function (O2 absorbing function). If the air-fuel ratio of gas that flows into the catalyst is rich of stoichiometry, the catalyst causes oxidation of unburned components, such as HC, CO, etc., by oxygen stored in the catalyst. If the air-fuel ratio of inflowing gas is lean of stoichiometry, the catalyst causes reduction of nitrogen oxides (NOx), and stores oxygen detached from NOx. Therefore, the three-way catalyst is able to efficiently remove unburned components and nitrogen oxides even if the engine air-fuel ratio deviates from a stoichiometric air-fuel ratio. Hence, the emissions control capability of the three-way catalyst increases with increases in the maximum value of the amount of oxygen storable in the three-way catalyst. Hereinafter, the amount of oxygen storable in the three-way catalyst will be referred to as “oxygen storage amount”, and the maximum value thereof will be referred to as “maximum oxygen storage amount”.
The catalyst degrades due to heat given to the catalyst or the poisoning by lead, sulfur and the like contained in fuel. It is known that as the degradation of the catalyst due to the poisoning progresses, the maximum oxygen storage amount decreases, and that as the degradation thereof due to heat progresses, the efficiency of removal of harmful exhaust components at a given air-fuel ratio decreases. It is also known that there is a strong correlation between the degree of progress of catalyst degradation due to the poisoning and the degree of catalyst degradation due to heat. Therefore, as indicated in FIG. 25, if a maximum oxygen storage amount Cmax of the catalyst is calculated or acquired, it becomes possible to determine whether the catalyst has degraded on the basis of the acquired maximum oxygen storage amount Cmax. That is, the maximum oxygen storage amount Cmax is a degradation index value that changes with the degree of degradation of the catalyst.
A catalyst degradation degree detecting apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 5-133264 detects the degree of catalyst degradation on the basis of the aforementioned finding, in the following manner. The apparatus switches the air-fuel ratio of gas upstream of a catalyst to a predetermined air-fuel ratio that is on the lean side of the stoichiometric air-fuel ratio or to a predetermined air-fuel ratio that is on the rich side of the stoichiometric air-fuel ratio. After that, the apparatus determines an amount of catalyst-passing gas that passes through the catalyst during a period from the switching of the air-fuel ratio until the air-fuel ratio detected by an air-fuel ratio sensor disposed in an exhaust passage downstream of the catalyst reaches the aforementioned post-switch predetermined air-fuel ratio. The apparatus further calculates an absolute amount of oxygen adsorbed and retained by the catalyst (i.e., a maximum oxygen storage amount Cmax) from the amount of catalyst-passing gas and the deviation of the post-switch predetermined air-fuel ratio from the stoichiometric air-fuel ratio, and detects the degree of catalyst degradation from the absolute amount of oxygen.
However, as indicated in FIG. 26, the maximum oxygen storage amount Cmax also changes depending on factors that affect the maximum oxygen storage amount, such as the catalyst temperature Tempc, and the like. Therefore, if it is determined whether the catalyst has degraded on the basis of, for example, a result of determination as to whether the maximum oxygen storage amount Cmax acquired without factoring in the catalyst temperature Tempc is greater than a catalyst degradation criterion value, a problem of unfavorable precision in determination arises.